Because of a lack of predictive tests, physicians must rely on patients' histories of previous drug reactions (or lack thereof) in prescribing future courses of medications. An adverse reaction to a drug which takes place while a patient is on a medication (particularly multiple medications) is often impossible to accurately diagnose. In addition, the nature of the underlying reaction may have an immunologic, toxic or pharmacologic basis. In vitro assays for predicting and diagnosing immunologically mediated adverse reactions to drugs would be of great value to practicing physicians. For example, while a radioimmune assay has been shown to be feasible for determining specific IgE directed against the major determinant of penicillin, no such assay exists for the minor determinants of penicillin nor for a whole host of other antibiotic and non-antibiotic drugs. When other than IgE antibodies are involved in causing immunologically mediated drug reactions, tests would be useful to define these situations.